LIDAR (light detection and ranging) systems involve emitting light and processing reflections of the light for applications such as laser-based remote sensing, image mapping, free-space optical communications, and other applications. For LIDAR to work, the source light or laser beam is steered to sweep across the target object or scene. Traditional laser beam steering system involved mechanical systems, which were large, expensive, and prone to failure. More recent systems include optical phased array (OPA) integrated circuit (I/C) chips, which have emerged as an alternative to traditional mechanical beamsteering devices such as spinning mirrors.
FIG. 1 is a representation of a prior art antenna system with equally spaced antennas. An OPA such as system 100 is traditionally composed of an array of periodically placed optical antennas. System 100 can steer an optical beam electronically without any mechanically moving parts, but has a very limited beam steering angle. In most cases the angle maxes out below 20 degrees. The limitation on the beam steering angle in the conventional OPA of system 100 is due to antennas 102 being placed periodically with a uniform spacing ‘a’ across the entire array. In such a periodic phased array, the emitted optical beam has aliasing beams, also referred to as grating lobes. The aliasing beams refer to a repetition of the beam signal energy every few degrees.
FIG. 2 is a diagrammatic representation of a prior art beam output of the antenna system of FIG. 1, with a main beam and several aliasing signals of similar energy. Diagram 200 represents the angular separation Δθ or difference in angle between the main beam and the nearest aliasing on its side. The angular separation can be given by the following equation:
      sin    ⁡          (      Δθ      )        =      λ    α  
In the equation, λ represents the operating wavelength of the optical phased array. When the beam is steered by the phased array, both the main beam and the aliasing beams move together, creating ambiguity when the beam is steered by more than Δθ. As such, in the presence of the beam aliasing, the angular steering range is limited to <Δθ. Since the antenna spacing (a) is typically much larger than the optical wavelength, which is typically on the order of 1 μm or micron, the available steering angle Δθ is usually limited to below 20 degrees. It is possible to increase the steering angle by shrinking the antenna spacing (for example, a system with a=1.3λ increased from below 20 degrees to approximately 50 degrees). However, there are practical limits on how small the antenna spacing can be shrunk, since providing high precision sub-wavelength antenna spacing may not be possible. Additionally, shrinking the antenna spacing results in an enlarged beam width, which significantly reduces spatial resolution.
Additionally, beam aliasing can be suppressed to some extent by randomly placing antennas, but such a design is impractical for a significant number of antennas due to the large amount of area needed to provide random spacing. Additionally, computing random spacing of antennas becomes computationally intensive when trying to reduce aliasing signal energy.
In diagram 200, the dashed line represents an envelope corresponding to the beam profile of a single emitter, illustrating the angular intensity distribution of a periodic phased array with many aliasing beams. The distribution of diagram 200 plots angle 202 against normalized beam intensity 204. Diagram 200 represents a curve for a Gaussian-shaped emitter design with 150-nm near-field Full-Width-Half-Maximum (FWHM). Main beam 210 represents the desired output signal, and aliasing 220 represents the repeated signal. It will be understood that many aliasing signals have significant amounts of energy relative to main beam 210, which reduces the resolution.
Thus, traditionally there is a tradeoff between resolution and beamsteering angle for solid state LIDAR systems. Furthermore, traditionally antenna design generates significant aliasing.
Descriptions of certain details and implementations follow, including a description of the figures, which may depict some or all of the embodiments described below, as well as discussing other potential embodiments or implementations of the inventive concepts presented herein.